Micro-electro-mechanical systems (MEMS) enables the development of state-of-the-art transducers, sensors, actuators, resonators, etc. used in a variety of applications ranging from consumer electronic market towards the automotive and even military electronic products. MEMS products offer the advantages of lower cost, better compatibility with high-volume batch fabrication, smaller space requirement, and higher reliability compared to the conventional electromechanical systems. The core of the MEMS products include precision micromachined electro-mechanical components that transduce physical, chemical, biological, etc. signals to electrical signals. Some of these micromachined components must have a direct physical contact with the outer world, as in the case of the gas flow sensors or pressure sensors. On the other hand, a great variety of the micromachined components, including but not limited to the inertial sensors, resonators, and infrared detectors, must be isolated from the atmosphere of the ambient in which they are operated. This isolation is necessary for both “forming a controlled operating atmosphere for the micromachined component” and “keeping these tiny components safe from the adverse effects of various factors including solid, liquid and/or gas contamination, humidity, and/or pressure variations”. Isolation of the micromachined components from the ambient is simply achieved by encapsulating them in hermetically-sealed packages.
The earlier examples of packaging MEMS components achieve hermetic sealing of every fabricated MEMS component individually, which increases the packaging cost due to the increased labor and time as well as reduced the process yield and reliability. An obviously better alternative is to seal the MEMS component at the wafer-level, which reduces the packaging costs significantly by minimizing the labor and time as well as increasing the process yield and reliability. Wafer-level packaging typically refers to the use of wafer-level MEMS processing techniques to form a capping element (either a layer or a wafer) on top of a sensor wafer that contains the MEMS components to be packaged. This way, all the MEMS components located on a sensor wafer can be encapsulated simultaneously. Encapsulation, however, is just the first half of the packaging process, whereas the second half is nothing but the transfer of the electrically conductive leads of the encapsulated MEMS component to the outer world without degrading the hermeticity of the encapsulation.
There are various methods reported in the prior art for the wafer-level encapsulation of MEMS components including techniques for lead transfer. Well known in the prior art is the use of glass-frit as the sealing material between a cap wafer and a sensor wafer, for which the leads of the encapsulated MEMS component is transferred laterally through the surface of the sensor wafer and creating a step-height below the glass-frit material, which must be sealed properly. Sealing step-heights up to few micrometers with glass-frit material is not a significant problem, since the glass-frit has a thickness typically more than 25 μm after firing. On the other hand, temperatures required for glass-frit bonding exceed 430° C., which may not only limit the number of compatible materials that can be used on the MEMS component but may also result in a high packaging stress. Moreover, glass-frit is a thick-film paste that has the risk of creating free-to-move frit particles inside the encapsulated cavity and contaminating the MEMS components, which may both reduce the packaging yield and long-term reliability. Finally, the hermeticity of the glass-frit is known to be worse compared to metal-based alloys used as sealing materials.
Another method in the prior art is the use of metal-based alloys as the sealing material (Au—In, Au—Sn, Al—Ge, Si—Au, etc.). These alloys generally provide better hermeticity compared to the glass-frit as well as require lower process temperatures typically in the range from 200° C. to 400° C. for various alloy materials and compositions. However, being electrically conductive, these sealing materials do not allow lead transfer through the sealing region, unless an additional insulating layer is used between the leads and the sealing material. Even with this insulating layer, the metal-based sealing material must still be capable of covering the step-height caused by the leads passing under the sealing region, which typically requires a sealing material thickness of a few microns or more. Such a thickness is not desired for metal-alloy based sealing materials due to the increased mechanical stress and also the cost of the thicker metal layers.
In the other examples of the prior art, the leads are transferred to the outer world using conductive feedthrough patterns that are machined vertically with respect to the surface of the sensor wafer. This way, a thinner sealing material can be used for sealing the MEMS component since the leads in this case do not cause a step-height under the sealing region as they are transferred to the outer world through a path that does not cross through the sealing region. Still, implementing the vertical feedthroughs increases the complexity and number of steps of the processes used to fabricate either the sensor or the cap wafers, or both. One of the difficulties with the vertical feedthrough processes is to achieve the sealing and the lead transfer in the same step, which requires precise control of the thickness' of the sealing material, sensor leads, sealing regions, vertical feedthroughs. Any offsets between some of these parameters may form “a properly encapsulated cavity without a successful lead transfer”, or “a successful lead transfer without an encapsulated cavity”. Another difficulty with some of the prior art using vertical feedthroughs is the void-free and hermetic filling of the via openings with a conductive material, which will form the vertical feedthroughs.
In summary, it is desirable to develop a method that achieves hermetic sealing of MEMS components by using either metal-alloy based sealing materials or even by some well-known bonding methods that do not require a sealing material at all. Moreover, this method should allow lead transfer using vertical feedthrough patterns that are formed by well-known MEMS etching processes, without requiring complex via-fill, trench-refill, or similar deposition-based techniques. A method with the abovementioned features would eliminate both the need for “high-temperature sealing processes” and also “refilling the vertical feedthrough patterns”, improve the process yield, reliability, and compatibility with different sensor processes, while reducing the packaging costs significantly.